Quick answer: you can't.
Unlike other product suites such as OpenView or Unicenter you cannot import MIBs.
But you can create UnDPs to monitor what isn't available in NPM out of the box. The hardest part is figuring out which OIDs you want to monitor.
If you do some searching through the forums you'll probably find the info needed to create your particular pollers, or you may even find a poller already out there and available for download.
While it's not possible to add MIBs directly into SolarWinds yourself. You can submit a case with SolarWinds support, send them the MIB files and request they be added to the MIB database. This is usually updated every 2 weeks I believe. While the addition of the MIBs doesn't mean SolarWinds will begin polling for that information, it does make it a bit easier to search for OIDs you want to monitor by having them in the updated MIB database you can get from the customer portal. This makes created Universal Pollers a bit easier.